This invention pertains generally to the field of nuclear medicine and particularly to improvements in apparatus for displaying images resulting from detecting radiations emitted by the body or an organ which has been infused with a radioisotope.
The improvements herein described may be used in conjunction with any of several known types of scintillation cameras which are more commonly called gamma cameras since they usually sense gamma radiation. A format display which will be described is useable with a variety of image acquisition and display systems.
A well known gamma camera is described in U.S. Pat. No. 3,011,057 to Anger. This camera has a large scintillation crystal disk which is disposed over the radioisotope infused organ and responds to absorption of gamma ray photons emitted from the organ by producing visible light scintillations. A closely packed array of phototubes is located behind the scintillator and their output signals, corresponding with scintillation events, are processed to determine the x and y coordinates of the scintillations and the energies of the gamma photons. The electric pulses which result from scintillations are fed to a pulse height analyzer. Pulses falling outside of a narrow energy range, called a window, are rejected and pulses within the window are considered valid and by suitable known electronic means, they produce a z signal coincident with the x and y coordinate signals.
The x and y coordinate signals are used to drive the beam deflection control circuits of a cathode ray tube on which the cumulative image of a large number of scintillation events is displayed. The cathode ray tube is unblanked only by the z signals so that only valid scintillations are displayed. Typical methods of developing suitable x, y and z signals are described in U.S. Pat. Nos. 3,697,753 and 3.919,556.
Images displayed on the cathode ray tube (CRT) screen are usually recorded with a photographic camera. There are known systems for displaying the images in different formats on the CRT screen and for recording the images correspondingly on photographic film. For instance, the CRT may be controlled so that the image occupies the whole area on the screen in which case the whole area of the film will be covered in one exposure. The CRT may also be controlled to display the images consecutively on smaller adjacent areas of the screen for being recorded correspondingly on the film. As many small images might be recorded as can be meaningfully recorded on the film and this depends on film size which is commonly 5 .times. 7 inches and 8 .times. 10 inches. In the apparatus to be discussed in detail hereafter, for example, as many as six columns and seven rows of small images may be produced on the screen in sequence and after each area is recorded on film the CRT deflection is indexed so that the next image will be recorded without double exposure. The exemplary system, besides permitting as many as 42 approximately 1 inch images on one 8 .times. 10 film, permits other combinations such as eight-on-one or four-on-one or fewer proportionately larger exposures on one film.
Insofar as is known, for high resolution systems, prior practice is to deflect the electron beam of the CRT electromagnetically and to focus the beam electromagnetically rather than to focus electrostatically. Electromagnetic focusing is used because it results in sharp focus which is important. However, electromagnetic focusing has disadvantages. It requires mounting a focusing coil on the neck of the CRT so it has all degrees of freedom for alignment and focusing purposes. That is, the coil must be able to translate in and out on the tube neck and it must be able to roll, pitch and yaw to effect proper focus and alignment. Getting focus and alignment correct at the same time requires so much skill and instrumentation that the adjustment is often made at the factory where the whole coil and its mounting is finally encapulated in resin so it cannot be adjusted in the field. If the CRT must be replaced, the permanently bonded focusing coil and deflection yoke must be thrown away too. Hence replacement is more costly.
If the deflection signal amplifiers do not have a high power rating the anode voltage on the CRT may be reduced to obtain the necessary deflection sensitivity. But the reduction in anode voltage is at the expense of losing some of the sharpness of magnetic focus so benefits of magnetic focusing are negated to some extent. If the electromagnetic deflection amplifier output current is low, the deflection speed is low. This has been compensated heretofore by slowing the gamma camera down or, in effect, stretching the camera pulses to get a longer flat line on the waveform to allow time for generating the z pulse. This, however, is at the expense of missing some of the scintillations and extends the time required for an exposure.
Another problem in prior art formatted gamma camera displays is to maintain the CRT beam focus even for images formed with the beam deflected far from the center of the CRT screen. Dynamic focus correction is, of course, known in the oscilloscope and television arts but dynamic correction circuits are costly since they require power supplies and components which must be as fast as the deflection amplifiers.
Another deficiency in prior art formatted gamma camera systems is the lack of a good mode for indicating to the operator in an unambiguous way which areas on the CRT screen and the film are available for use, which have been exposed and which formats are still available for image exposures of various sizes so as to be certain that double and overlapping exposures will be avoided and that available areas will not be wasted.
Still another problem in prior art gamma camera imaging systems is that of using the same high deflection coil voltage and power for the small beam deflections required for multiple small format images as is used for a single image that fills the entire face of the CRT.
Yet another problem is in the schemes which have been used heretofore for setting the voltages that govern the intensity of the CRT beam and, hence, the film density during a diagnostic study and for calibrating the intensity voltages between studies to compensate for loss of brightness resulting from aging of the phosphor screen in the CRT. Known prior practice is to use a light sensor which views an area of the tube screen that is dedicated to developing a test signal indicative of phosphor brightness in that area but not in the areas where images are normally formed so it is possible that the test will not simulate true operating conditions.